I'm going to sneak into the whole killing morality discussion if no one minds too much.

Here's my humble little thoughts on it:

Killing the joker isn't a moral action. Killing is never moral. However, it can be a good solution. I don't want Batman to start killing people, but can you seriously say that killing the Joker would be a horrible thing?

To put things in a more realistic perspective (well, sort of), think of this scenario:You're driving to work one day, and you hear on the radio that a convicted killer has escaped from jail somehow and has been running around killing and seriously wounding multiple people and has been able to avoid the cops so far. They don't know where he is, and although you are concerned, you continue to drive to work. When you arrive, you find a dead cop lying halfway in and out of the front door. You cautiously glance inside and see several other dead bodies, several of whom you recognize as coworkers and friends. The killer is standing in the middle of the reception area with his back toward you, firing at various screaming people. You notice that the dead police officer has a gun in his hand. So here's the question: Do you run away from the building and call the cops, or do you call the cops and then pick up the gun and at least try to kill the man. This is assuming that you are not proficient with firearms, and you most likely cannot get in a shot that merely incapacitates the killer without killing him.Granted, this frightening situation may not give you much chance to think through your actions, and running away would be a natural reaction. However, assuming that you have some clarity, could you really completely rule out the possibility of picking up that gun and trying to end the massacre? Can you justify saving one life while risking countless others? If you succeed in cutting the killer down, you may feel immense guilt (hopefully at least some anyway - killing shouldn't be an easy act), but I would assume that you would rather feel an immense burden over you for the rest of your life than seeing more people die.

And as someone said before, in DC comics things are very different. People don't stay in jail for good. We shouldn't have killing vigilantees today because people DO stay in jail and they don't have super powers that they can use to break out and kill hundreds of people.